Concentration recorder



May 30, 1944- J. A. VAN DEN AKKER CONCENTRATION RECORDER 9 Sheets-Sheet'l Filed Sept. 2, 1941 zNvENToR. vf/zafafs J?. am den afi/afer BY 52m/m,Pmf @wif/wm May 30, 1944. J. A. VAN DEN AKKER CONCENTRATION RECORDERFiled Sept. 2, 1941 9 Sheets-Shea? 2 INVENTOR.

May 30, 1944- J. A. VAN DEN AKKER 2,350,001

CONCENTRATION RECORDER 2742 INVENTOR.

#YJ/afwas a?. Van dem QQ//ew BY Sm j PM) +Mw May 30, 1944- J. A. VAN DENAKKER CONCENTRATION RECORDER Filed Sept. 2, 1941 9 Sheets-Sheet 4@grass- May 30, 1944- J. A. VAN DEN AKKER 2,350,001

CONCENTRATION RECORDER Filed Sept. 2, 1941 9 Sheets-Sheet 5 Q INVENTOR.f// cames d?. Van cie/L @QZ/0416,,

May 30, 1944- J. A. VAN DEN AKKER 2,350,001

CONCENTRATION RECORDER Filed Sept. 2, 1941 9 Sheets-Sheet 6 sQ g May 30,1944- .l. A. VAN DEN AKKER CONCENTRATION RECORDER Filed Sept. 2, 1941 9Sheets-Sheet 7 .w mi uw wm, 1V \m\\ w@ m Mv am P, l QQ). l i JT Q l. wwwNN NNW\ I|. L 1|| mll| .|||l\. l.|||ll|||l.l no fl :L o in llllll Illlllllwll 0 I llmlbwxllllll \N\ .rl u l/J Ahi Jal U W.mllllllllQUNlvNwl .l \l .m.%\ lV.. wullllllll 1%. lTl UQ. RQ lll lllllnnlllllnllln ll Tl l lllll Il ..ll| ll ll T@ vlllllnll. llllllLN lh,MEW: M Ww l llplL nnHHHHllh Hlld l++ ll l N@ l l` luz; l ll l. l l llllv U l 1 al; llll ,All li ill@ /l l\ l,l www@ llllllllrllllHNlHWMlllll//lll/lhw 9 Sheets-Sheet 8 May 30, 1944. J. A. VANDEN AKKl-:R

CONCENTRATION RECORDER Filed Sept. 2, 1941 im .SN @wml May 30, 1944. J.A. VAN DEN AKKER CONCENTRATION RECORDER Filed Sept. 2, 1941 9Sheets-Sheet 9 BY szmd, fm@ VLM w :fw E P n l@ 5 f .y .m ed @may m am ifW 7. i 6 M M 6 0 Z J F 6 w mm W 1 w j 2 Z oO di W 9\ 3W Z N Y \/rPatented May 30, 1944 l UNITED STATES PATENT OFFICE CONCENTRATIONRECORDER Johannes A. Van den Akker, Appleton, IWiI.. as-

signor to The Institute of Paper Chemistry, a corporation oi' WisconsinApplica-tion September 2, 1941, Serial No. 409,174

9 Claims. (Cl. Z50-43) My invention relates, generally, to apparatus Aspeciiic object of this invention is to provide ior measuring andrecording concentrations of' an inexpensive and practical concentrationresubstances which have the property of absorbing corder which isparticularly adapted to acculight energy of certain wavelengths, theapparately measure and record small concentrations ratus beingparticularly adapted to measure and of ozone. record smallconcentrations of such substances. vAnother objectof this invention isthe provi- As will hereinafter appear, the disclosed embodision of aninexpensive light conversion and selecment of the invention includes anew and useful tion system which makes possible the use of a light.conversion and selection system, photomecommercial light source or lampwhich emits ter, and electrical circuit, each of which serves as 1oiight in a plurality of wave bands as a source 0f one of the componentparts of the concentration light energy in a single one of the emittedbands. recording apparatus, but which are not limited This lightconversion and selection system is parsolely to use with this apparatus.vticularly adapted for use as part of my improved It is a knownscientific fact that certain subconcentration recorder but may be put tocertain stances have the property of strongly absorbing l5 otherimportant applications light of certain wavelengths, particularly in theIn obtaining maximum sensitivity of response ultraviolet region of thespectrum. Examples of in the apparatus of the present invention, angases and vapors having such a property are electrical bridge circuithas been provided which ozone, sulphur dioxide, mercury vapor, chlorine,I produces large changes in wave form in response perchloroethylene,phosgene, and a fairly large to relatively small changes in light iiux.'I'his group of other gases and vapors. Examples of bridge circuit isparticularly adapted to control solutes which in solution have theproperty of sas-lled electric valves, and although it is Derabsorbinglight of certain wavelengths are poticularly adapted for use inconnection with contassium dichromate, amino dlsulfonic acids, andcentration recorders embodying my invention, it sodium benzoate. Ingeneral, light energy abmay be advantageously employed in connectionsorption by these various substances is proporwith other applications.tional to, or bears a definite relation to, their The nature andPrinciples of my invention may concentration. be more fully understoodfrom the following de- The present invention, making use of this lighttailed description of an ozone concentration reabsorption principle,provides aninexpensive and corder which forms one embodiment of theinpractical concentration recorder for accurately VentiOn. Extendedtests have Shown that this measuring and simultaneously recordingconcenozone concentration recorder is commercially trations ofsubstances having this particular Practical and Will accurately measureand record. property of appreciable absorption for nght radismallconcentrations of ozone in a very satisfacation of certain wavelangths.Although there is tory mannersome literature treating in a general wayon this In the interests 0f Simplification. the detailed phenomenon oflight absorption by certain subdeSCriDtiOn 0f the Selected embodiment ofthe instances, and some progress has been made in de- VentiOn Will bepreceded by a brief diSeuSSiOn 0f veloping apparatus for only measuringor indithe basic principles 0f operation thereof taken in eating theconcentrations of suchsubstances deonneetiiln With Suitable diagrammaticdrawings. pending upon their particular light absorption In the drBWinZSproperty, it does not appear that there has been Fig. 1 iS a diagram 0fB Simple basic System by any substantial progress made in developingapwhich the concentration of a substance may be paratus for bothmeasuring and recording in a measured depending D011 its Property ofabsorbcontinuous manner the concentrations of such ing iight radiation0f e. particular Wavelenth;

substances depending upon their light absorp- Fig. 2 is a diagram of asystem similar to that tion properties. shown in Fig. l, but employingthe light con- Accordingly, the object of my invention, gen- Version andSelection System o! the present inerally stated, 1s to provide aninexpensive, pracvention instead of a monochromatic illuminator; ticalapparatus for measuring and continuously Fig. 3 is a comprehensivediagrammatic view of recording in an accurate 'manner the concenaconcentration recorder embodying the features tration of certain gases,vapors, and solutes hav- 0f the Present invention; `ing the property ofabsorbing to an appreciable Fig. 4 is a view taken generally on line 4 4of extent light radiation of certain particular wave- Fig. 3; lengths.5s Fig. 5 is a diagrammatic view oi a photometer aperture forming animportant part of the present invention;

Fig. 6 is a mathematical diagram by which the principles of constructionV'and design of theA photometer of Fig. may be explained;

. Figs. 7, 8 and 9 are electrical diagrams or wave form sketches bymeans of which certain electrical observations important in connectionwith the electrical bridge circuit of the present invention may beexplained;

Fig. 10 is a diagram of a light-sensitive elecltrical bridge circuitadapted to produce large changes in wave form in response to smallrelative changes in illumination of a pair of photocells and forming animportant feature of my present invention;

Fig. 11 is a diagram or sketch showing changes in wave form produced bythe electrical bridge circuit of Fig. 10;

Fig. 12 is a diagram showing an electrical control system by which areversible motor may be controlled in response to changes in wave formproduced by an electrical bridge circuit embodying the essentials of thebridge circuit of Fig. 10;

Figs. 13s and 13b are horizontal sectional views of a successfullyoperated and tested ozone concentration recorder instrument madeinaccord` ance with the principles of the present invention, Figs. 13s.and 13b together forming a complete horizontal section through thisapparatus;

Fig. 14s isa vertical sectional view taken on line Ida-Il of Fig. 13a:

Fig. 14h is a vertical sectional view taken on line llt-I4 of Fig. 13b;

Fig. 15 is an elevational view of a mercury vapor lamp taken on lineI5-I5 of Fig. 13s;

Fig. 16 is a vertical sectional View taken on line IG-IG of Fig. 13a;

layer of ozone gas in centimeters. Thus transmittance T of a quartz cellcontaining ozone gas may be expressed by the equation T= L= 1 ow (2) llo a=a/p= P19/0.002 14=69,700 cm3/g.

From this, p may be expressed by the equation :v

Fig. 17 is a. circuit diagram of the electrical control system for theozoneconcentration recorder instrument in Figs. 13a, 13b, 14a and 14n;and

Fig. 18 is a diagram of a modied form of light-sensitive electricalbridge which has been advantageously substituted for the electricalbridge oi.' the electrical control system shown in Fig. 17. In theelectrical bridge of Fig. 18 the photoelectric cells are or thephoto-emissive type, whereas in Fig. 17 the photoelectrlc cells are ofthe blocking-layer type.

Ozone gas has the ability to strongly absorb ultraviolet light ofcertain wavelengths. In particular, the absorption by ozone gas of lightradiation in the 253.7 mmu line of the mercury spectrum is exceedinglystrong, the extinction coeiiicient being 149 cm.1, under standardconditions of temperature and pressure. That is, a layer of ozone havinga thickness of 1 cm. will, under standard conditions of temperature andpressure, reduce the intensity 01,253.7 mmu radiation to 1/10149. ALuchli (Z. Physik, 53:92 (1929)), reported extinction coemcients a forozone `gas under standard conditions oi normal pressure and temperature,as follows:

Table I wavelength; 237.8 248.2 253.7 265.2 280.4 296.7 312.5

The extinction coelcient a is defined by the equation:

(l) 1:1010-ah p=(1/ah) logic (l/T) Applying the foregoing data, it willbe seen that the system shown in Flg. l can be used to measure theconcentration of ozone gas. Referring to Fig. 1, a mercury arc,indicated diagrammatically at I0, is used as a. source of 253.7 mmuradiation. 'I'he radiation of 253.7 mmu wavelength is isolated from`radiation of other wavelengths emitted from the arc 10 by a quartzmonochromatic illuminator, indicated diagrammatically at II. Anabsorption cell I2, through which ozone gas may be circulated, is placedin line with the monochromatic illuminator Il so as to receive 253.7 mmuradiation transmitted therethrough. Radiation passing through the cellI2 is received by a photocell I3 adapted to be sensitive to ultravioletlight through the use oi a uorescent screen' (not shown) which may beprepared, for example, by dipping plain transparent Cellophane in asolution of Uranine B dye. The system is completed by a galvanometer,indicated diagrammatically at I4, having its terminals connected withthe terminals of the photocell I3.

In operation, as 253.7 mmu radiation passes through the absorption cellI2 its intensity is reduced to an extent depending upon theconcentration oi' ozone therein. In turn, the photocurrent produced bythe photocell depends upon the intensity of the 253.7 mmu radiationincident thereon. Accordingly, the amount of deilection of thegalvanometer Il will depend upon the concentration of the ozonecirculated through the absorption cell I 2.

To calibrate the apparatus, a table of gal" vanometer deflections withcorresponding ozone concentrations may be prepared for the system ofFig. 1. as follows:

As above stated, the mass of ozone per cubic centimeter pis related tothe transmittance T in accordance with Equation 3 peu/ah) legio (1/T)Now, since a and h are constant, and T is proportional to the deilectionD of the galvanometer when Io is constant, we have equation- (4)p=1cilog1o (l/D) The value of k1 may be determined by observ- 334.1 mmuing the deection D1 for a known concentration p1 of ozone, thensubstituting these known values in Equation 4 and solving for k1. Havingdetermined kl, a calibration table for the system may be calculated bysubstituting different values for D and solving for corresponding valuesfor p. By using absorption cells of different gas thickness h, ozoneconcentrations in different ranges can be measured with good accuracy.

Although `the ozone concentration indicating system oi' Fig. 1 vissuccessful in operation from a scientiilc standpoint, it necessitatesthe use oi the monochromatic illuminator il. Accordingly,

to eliminate this piece of apparatus, the system shown in Fig. 2 wasprovided.

Referring to Fig. 2, a mercury vapor lamp Il is shown which serves as asource of ultraviolet light for the system. A Westinghouse Sterilamphaving an M-shape was found to serve Table II Wavelength Relative (mmu)energy 253. 7 l1. 3 265. 2 0. 032 E0. 4 0. 011 E9. 4 i). 016 296. 7 0.065 302. 2 0. 043 312. 9 0. 34 365. 4 0. 30 404. 7 0. 36 435. 9 l. 09546. 1 0. 60 578. 0. 13

A quartz absorption cell i0 through which ozone gas may be circulated isdisposed ln front of the lamp Ii, and a glass plate l1 coated with athin layer or deposit of cadmium borate phosphor is placed in line withthe cell I8 to receive radiation transmitted therethrough. The cadmiumborate phosphor is strongly excited by radlation of 253.7 mmuwavelength, is weakly excited by near ultraviolet light, and receives noexcitation ,at all from visible light. When excited by ultraviolet lightalone, the cadmium borate phosphor fiuoresces in the orange and red partof the spectrum.

According to R. N, Thayer and B, T. Barnes (J. Optical Soc. Am. 29:131(March, 1939)), the spectral excitation curve of cadmium borate phosphoris negligible for wavelengths greater than approximately 380 mmu and, ingoing toward shorter wavelengths, rises very steeply, being verysubstantial at a wavelength oi' 253.7 mmu. When the values for relativeenergy given in Table I are multiplied by the spectral excitation valuesgiven by Thayer and Barnes, the excitation due to 253.7 mmu radiation,and excitations due to neighboring lines are obtained as given in the Itwill be seen from Table III that' the relative excitation of thefluorescent plate i1 due to 253.7 mmu radiation is 11,300, whereas thatdue to the neighboring lines in the ultraviolet has a total of onlyabout 153. Computation shows that the amount oi' excitation due toradiation emitted from the lamp l5 in the neighboring lines-ot themercury spectrum is only 1.4 per cent oi that due to 253.7 mmuradiation. As can be seen from Luchli's data in Table I above, theseneighboring lines are somewhat absorbed by ozone but, assuming theysuiIered no absorption (this enables computation oi' maximum error), anidea of the error involved can be obtained by taking a transmittancevalue T of the ozone in the absorption cell IB as equal to 0.500. Theobserved transmittance would then be [new/21er@ (5) 11,300+153 05067 Ifthis corresponds to a concentration of ozone of 1.00 per cent, theobserved concentration would be 0.98 per cent. This is only an error of2 per cent oi the value of the concentration itself. When theconcentration of ozone c=0.5 per cent, T=0.707; the' observed T would be0.711 and ac=0.008, or about 1.6 per cent of the concentration itself.These discrepancies are satisfactorily small and may be compensated forin calibration of the instrument.

Fluorescent radiation from the plate Il, which is most strong in the redportion of the spectrum, is filtered through a filter I8 before itreaches a photocell I9. The filter Il may be a Wratten No. 25 illterwhich excludes or absorbs all direct radiation of wavelength less than580 mmu, but freely passes radiation of longer wavelength. Hence, theresponse of the photocell I9 due to the Alines 578.0, 546.1, 435.9 mmu,etc., in the system,

' spectrum would give rise to a concentration error of a smallnesscomparable with that under.2.0

per cent discussed above in connection with Equation 5. Instrumentcalibration may be relied upo`n to take care oi' this small error.Accordingly, the photocell I9 (which may be of the blocking-layer type)will give rise to a photocurrent which is primarily dependent upon andresponsive to the intensity oi 253.7 mmu radiation.

The deflection of a galvanometer 20 connected in series with thephotocell I9 will be proportional `to the photocurrent produced thereby.With a .uniform amount of 253.7 lmmu radiation being supplied to theabsorption cell I6 from the mercury vapor lamp l5, the photocurrent willbe proportional to the intensity of the 253.7 mmu radiation transmittedthrough the absorption cell I6, and accordingly will vary inversely withthe ozone concentration therein. To calibrate the apparatus, a table ofgalvanometer deflections and corresponding values of ozone concentrationmay be prepared for the system of Fig. 2 according to the method asdescribed above in connection with the system of Fig. 1.

The systems described above in connection with Figs. 1 and 2 of thedrawings serve only to measure or indicate ozone concentrations. Suchapparatus, while perhaps suitable for measuring the concentration ofozone in various gas samples from-time to time, is inadequate sincethere are many applications and instances where it is necessary tomeasure and record the concentrations 4 fozone and other gases andvaporsin a continuous manner. In Figs. 3 and 4 of the drawings, such asystem for measuring and recording ozone concentrations in a continuousmanner is shown. The general arrangement and broad principles ofoperation of my concentration recorder may be conveniently understood bya brief, general description thereof taken in connection with Figs.

3 and 4, while 'a detailed description of the various operating partsand elements thereof can be advantageously given below.

Accordingly, referring to Figs. 3 -and 4 of the drawings, thereis`shown' at 25 a mercury vapor lamp, which may be in the form of anM-shaped Westinghouse Sterilamp, and which serves as a source ofultraviolet light. 'I'he light energy dis- -tribution characteristics ofthe vapor lamp 25 l have been described above in connection with themercury vapor lamp |5 of Fig. 2. A pair of quartz-window absorptioncells 26 and 21 are placed to the right of the lamp so as to receiveultraviolet light therefrom. The rays of radiation from the lamp -25 tothe cells 26 and 21 are indicated in broken line. The upper absorption y.cell 26 serves as the active cell through which ozone gas may'becirculated. When the concentration of ozone or other gas or vapor in airis being measured it is not necessary to provide the lower absorptioncell 21. However, when the concentration of a solute in aA solvent isbeing measured, the clear solvent should be placedin the lowerabsorption cell 21.

Aplate 28 is placed a short distance to the "right of the absorptioncells 2s and 21, this plate 30. A photometer member 3|, the constructionand design of which is described in detailfhereinafter, is shiftablymounted in front of the upper opening 29. The photometer member 3|andthe opening 29 together form a variable photometer` aperture formeasuring light passing through the upper absorption cell 26. vAnadjustable shutter member 32 is provided in the back of the lower Window3|) which serves to adjust the Width thereof.

lines of the mercury spectrum, so that the response of the upperphotocell 31 is substantially entirely due to the amount of 253.7l mmuradiation passing the photometer. 'I'he manner in which this light'conversion and isolation is accomplished is described above inconnection with Fig. 2. Likewise, the fluorescent plate 34 and filter36, forming the lower set of light conversion and selection elements,convert the ultraviolet 253.7 mmu radiation passing through thoseelements into visible light, so that the response of the lower photocell33 will be substantially entirely dependent upon the amount of 253.7 mmuradiation passing through the window 30.

The photocells 31 and 33 comprise part ot an electrical control systemfor controlling a reversible motor 45. The control system for t'ne motoris indicated diagrammatically in Fig. 3 by the box 46. The details ofthis electrical control system will be iully described below. Theelectrical control system and the reversible motor 45 are energized fromthe source of alternating' current as indicated by the circuit diagramin Fig. 3.

" The reversible motor 45 operates the photoml eter by shifting thelphotometer member 3| one side of the worm .wheel 49 and a second A pairof the light conversion and selection systems described in connectionwith the system of Fig. 2 of the drawings are incorporated in theconcentration recorder of Fig.l 3; that is to s'ay, the system orcombination formed by the iuorescent plate l1 andthe lter 'I8 of Fig. 2.In Fig. 3, a -cadmium borate phosphor coated plate 33 is positioned tothe right of the upper opening 29, while a similar fluorescent plate 34is positioned to the right of the Window 30. The fluorescent plates 33and 34 may have the same excitation characteristics as the iluorescentplate |1 de-l scribed in connection with Fig. Y2; A pair` of red lters35 and 36 are placed to the right and inA back of the iluorescent plates33 `and v34 respectively, so as to receive uorescent light therefrom.Wratten No. 25, or "A" iilter type, having the absorptioncharacteristics described -in connec- -tion with filter I8 of Fig. 2.- Apair of photocells 31 and 38 are placed behind the iilters 35 and 36,respectively, so asto receive light passing therethrough. A horizontalpartition 39 serves to separate the upper set of elements from the lowerpulley 52 mounted to the left of the photometer member 3|. One end ofthe cord 50 is fastened to the upper right hand corner of the photometermember 3| at 53, andthe other end 0i' the cord 50 is fastened to theupper left hand end of Y the photometer member 3| as indicated at 54.

, The pulley 52 is carried on a bell-crank lever 55 which is pivotallysupported at the point 54.

\ A spring 51 connected to the lower arm of the Thel lters 35 and 36maybe of the.

bell-crank 55 and a rigid part'of the recorder,

serves to tension the pulley 52 toward the left.

hereby maintaining the cord 50 in a, taut con.

t on.

'I'he record of the concentration recorder is made on a time-drivenchart 54 by a stylus or pen 59 carried on the end of an arm. extendmgfrom the left hand side of the photometer member 3 In operation'I theconcentration recorder is ilrst adjusted for zero reading. When used tomeasure and record the concentration of ozone in air, this initialadjustment is made with the upper and lower absorption cells 26 and 21containing air only. It will be understood that the photometer member 3|has diierent positions ranging from that corresponding to zero ozonevwhen the member 3| is in its zero position. After this adjustment hasbeen made, the recorder is in condition to measure and record ozoneconcentration. i

When the concentration recorder is put lnto operation and ozoneiscirculated through the upper absorption cell 2l, the intensity of253.7 mmu radiation transmitted lthrough this cell will be reduced inaccordance with the concentration of ozone therein. As a result, theamount of 253.7 mmu radiation passing through the photometer willbecorrespondingly reduced, and with the photometer member 3| in its zeroposition, will not be sufficient to balance the amount of the sameradiation passing through the lower window 3l. Accordingly, the controlsystem will Jbe un balanced, and the reversible motor will be operA atedso as to shift the photometerlmember 3| from its zero position to aposition where a suf-'30 flclent amount of the 253.7 mmu radiation ispassed to balance that passingthrough the lower window 30. Thereafter,as the concentration oi' the ozone circulated through the absorptioncell 23 changes from time to time, the photometer member 3| will beshifted from one pusition to another so as to keep the system inbalance. 'Ihe concentration will be recorded on the chart 53 by theelement 59 which shifts with the photometer member 3|. The details ofthe photometer and the electrical control system are described below.

It will be seen that the photometer system of the concentration recorderis of the so-called null balance type. That is, a definite relationshipis maintained between the amount oi' 253.7 mmu radiation passing throughthe photometer comprising the photometer member 3| and the opening 29,and the amount of the same wavelength radiation passing through theadjustable lower window or aperture 30. 'I'he great advantage affordedby the employment of the "null balance type oi' photometer system isthat the accuracy of the concentration recorder is not affected bychanges in output of radiation from the vapor lamp due to changes inline voltage.

The concentration recorder will thus accurately measure and recordindependently of variation in output from the lamp 25.

Photometer system For accuracy and convenience, it was found It will benoted that the variable aperture member 3| has ythree similar elongatedopenings 5|, 62 and 33 of exponential shape formed therein. Threelopenings 3|, 82 and 33 are used so as to reduce photometric error. If asingle large opening were used instead, it will be seen on referring toFig. 3, that the points of uorescence on the plate 33 would move from acentral area to cover most of the plate 33 on shifting the member 3|from its zero position shown in Fig. 4 to its opposite position wherethe greatest amoimt of light is passed thereby. This tanning or wideningout of the excited fluorescent area might give rise to a photometricerror, since the response of the upper photocell 31 to fluorescent lightof given source intensity depends upon the position of the source on thefluorescent plate 33. Theoretically this error would be minimized to anegligible value if a large number or equally spaced openings havingparallel axes were cut in the photometer member 3|. Since such anarrangement would be impractical, the three elongated openings 6 I, 52and 63 were chosen. The photometric error with the three openings hasbeen found to be sufficiently small so that the overall accuracy of therecorder is not impaired.

The calculation of the shape of the openings 3|, 02 and .63, may beexplained as follows, with reference to Fig. 6:

a=width"of the opening 23, w=total opening in member 3|, let x=distanceon recorder chart=distance of motion of the member 3| from the originor.

zero position. We know that the transmittance of the ozone is: T'=10i,where a=extinction 69,700 cmP/g. t=thickness of ozone in cm.c=concentration of ozone in g./cm.3.

Since we desire a linear scale, c=kax.

The photometric method calls for constant ilux of 253.7 mmu radiationgoing to the fluorescent plate; hence, calling xm the maximumdisplacement in operation of the photometer member 3 I, we may writewhere T'mm. is the value of T' at :rm and at maximum recordedconcentration. The object is to nnd a function of :c which will satisfythe above integral equation. It is easy to show that the solution is asimple exponential function:

Put j(x)=AeB. Then, substituting and integrating, we obtain:

VOl'

desirable to provide a photometer having a linear scale so designed thatthe amount of shift of the photometer member 3| from its zero positionrequired to balance the system should be a linear or straight-linefunction of the ozone concentration in the absorption cell 26. Referencemay be made to Figs. 5 and 6 of the dr-awings -for a detaileddescription oi' the variable aperture member 3|. (Fig. 5 is a rear viewof the photometer member 3| in respect to its position shown in Figs. 3

and 4 with the opening 29 indicated in broken outline.) .Y

coeillclent: Y

Finally, the equation for the total opening in the exponential openingbecomes (9) 1(1) blo-keatlrmw-z) We may select as the following vvaluesof constants, values which were actually used in the design of thephotometer member of asuccessfully operated ozone recorder instrument tobe described hereinafter:

based on weight for dry air at 21 C., 75 cm.v

and the results are presented .in the following table:

Table IV .The values given in Table IV are equally divided between thethree parallel elongated openings Gl, 62Y and 63. In the photometermember designed for the working embodiment of my invention, the axes ofthe openings 6l, 52 and 63 are each separated by a distance of 0.85 cm.'Ihe openings 6|, 62 and 63 aresomewhat longer than 3.5 inches (xm) soas to permit overplay at zero and maximum concentrations. (A distance onthe 'recording chart of exactly 3.5 inches was chosen for lthe range inozone concentration of from 0.0 to 1.0 per cent.) As indicated in theforegoing theory, all calculations have been based on concentration ofozone expressed in grams per cubic centimeter '(g./cc.). Percentagecomposition, based on weight, is made relative to the density of dry airat 21 C., 75 cm. Hg atmospheric pressure.

In making the above calculations it has been assumed that all of therays of 253.7 mmu radiation passed through the absorption cell in adirection perpendicular to the quartz windows thereof. In any practicalarrangement this condition cannot be completely met and, therefore, thevincident rays are not perpendicular to the absorption cell but actuallycover a rance of angles from zero degrees to a certain maximum valuedependent upon the size of the licht source, the size of the absorptioncell, etc. On the basis of sound theory the error due to this deviationfroin'the ideal picture may be calculated. Such calculations have beenmade for the dimensions used in the ozone recorder instrument describedhereinafter. They showed that the actual cell thickness (inside width)should be 0.98 of the theoretical thickness. Thus, with a theoreticalinside cell thickness t of 0.400 cm., theactual inside cell thicknessshould be- 0.4 0.98 or 0.392 cm.=0.1543 inch Electrical control systemThe electrical control system employed in connection with theconcentration recorder diagrammaticallyshown in Figs. 3 and 4constitutes an important feature of the invention, andinvolves the useof a novel type of electrical bridge circuit forcontrolling gaslledelectric valves. 'I'he principles of my electrical control circuit aredescribed below inconnection with Figs. 7 through 12.

Referring particularly to Figs. 7, 8 and 9, the following observationwas made in experiments with a single blocking-layer photocell on theinput of an oscilloscope. With the photocell B5 connected across theinput of the oscilloscope 56,

- as shown, and exposed to an intense source of light showing a. strongperiodic variation' in intensity, a voltage wave of insignificantamplitude was observed on the fluorescent screen 6l of the oscilloscope.However, when the unearthed terminal of the input was connected to ashort antenna, as indicated by the' broken line, to pick up thesixty-cycle wave, the wave form on the iuorescent screen 61 changed verymarkedly as sketched in Figs. 8 and 9. In Fig. 8 the sixtycycle sinewave is shown which is obtained when the photocell 651s in the dark.However, when the photocell 65 is exposed to low illumination of eithersteady or fluctuating intensity, the sine wave is materially changed inone half part of the cycle as shown in the sketch of Fig. 9. The brokenline part of the sketch in Fig. 9 indicates the normal sine wavepattern. The explanation of the change in sine wave as shown in Fig.9,is that illumination decreases the shunt resistant across theinputterminals of the oscilloscope GBduring one-half of the cycle. Althoughit was known that the internal resistance of a blockinglayer celldepended upon theintensity of light falling upon the photocell, it wassurprising to find that the sensitivity of change of wave form to lightwas very much greater than the direct photoelectric e'ect, where arapidly fluctuating source of light is involved, and one isV interestedonly in the alternating current' component. By way of explanation, itappears that the direct photoelectric effect was impaired by the lowimpe'dance of the blocking-layerphotocell.

It was found that the above observation of the sensitivity of wave formto light. of a blocking-'- layer photocell could be used to provide anelectrical bridge circuit which was very sensitive to small relativechanges in light ilux from two light`sources so as to produce largechanges in wave form. Such a bridge circuit is shown diagra-mmaticallyin Fig. 10 of the drawings.

Referring to Fig. 10, it will be seen that the electrical bridge circuitcomprises four legs. One pair of the legs comprises adjustable impedancedevices indicated diagrammatically at 'l0 and 1l, and the other pair oflegs each include a blocking-layer photocell indicated diagrammaticallvat l2 and '13. 'I'he photocells 'I2 and 13 should be matched forsensitivity. internal resistance and capacitance. Each of the pairs oflegs are connected in series circuit relation and the pairs of legs arethen connected in parallel circuit relation. as shown. The conductors 14and 15 serve to vconnect the bridge with a source of altexnating currentindicated diagrammatically at 16. Adjustable impedance devices indicateddiagrammatically at 8l and 11 may be connected in shunt relation withthe photocells 12 and 18 respectively. The adjustable impedance devicesIl and 11 may each include an adjustable registor and an adjustablecapacitor connected together either in series or parallel circuitrelationship. The input of an electronic amplifier indicateddiagrammatically at 18 may be connected across the common connections 18and Il between each of the pairs of legs, as shown. The output of theamplifier 1l is connected to the input of an oscilloscope indicateddiagrammatically at 8|.

Many interesting eil'ects were observed in connection with thiselectrical bridge arrangement. When an alternating current voltage,within a range to which the electrical bridge circuit was sensitive, wasapplied across the bridge from the source 18, and the bridge balancedthrough the manipulation of the impedance devices 1l,

1| and 11,A the bridge circuit became very sensitive to small relativechanges in iight flux gathered by one of the photocells. When the cells12 and 18 are in darkness, the bridge circuit may be balanced so as toobtain the residual wave 82 sketched in Fig. 1l. This residual wave isindicated by the full line having loops of equal amplitude above andbelow the reference line.

Theoretically. if the photocells 12 and 18 were identical and completelymatched for sensitivity, internal resistance and capacitance. theresidual wave 82 would be a straight line.

The change in wave obtained when one of the photocells receives anincrement in light flux or illumination not accompanied bv a similarincrement received h v the other photocell. is indicated by the waves 83and 84. the wave 88 being designated with dots and dashes. whereas thewave 84 is designated with uniform dashes.

Referring to the sketch of Fig. 1l it will be seen from the waves 83 and84 tht peaks 360 electrical degrees apart increase. and alternate peaks(180 phase difference) decrease. when one of the photocells receives anincrement in illumination and. viceversa. the alternate peaks rise andthe former set ofv peaks decrease'when the other photncell receives anincrement in illumi` nation The wave 83 Vrepresents the wave obtainedwhen one of the photocells 12 or 18 receives the lgreater illumination.while the wave 8| represents the wave obtained when the-other Dhotocelireceives the greater illumination. When light flux to the two cells 12and 18 is greatly increased. but balanced (through only change inrelative flux). the residual wave 02 shown in the sketch of Fig. l1 doesnot change greatly in either amplitude or phase, the change in phasebeing verysmall; moreover, the phenomenon is not significantly alteredwhen a strongly pulsating light source (1-tube G. E. Mazda iluorescentlamp) is used.

It was found that the value of the alternating current voltage impressedon the bridge circuit was an important factor. When this voltage was thewave forms shown in connectionwith sketch of Fig. 11 were againobtained.

Itis apparent that the wave form patterns shown in the sketchesA in Fig.11 are well suited for direct control of electric valves of thegasfllled type (Thyratron or Grid-glow tubes). The grids or controlelements of the Thyratrons should be operated together and the platesoperated apart by 180 electrical degrees.

In Fig. 12 of the drawings one type of electrical control system for theconcentration recorder of Fig. 3 is shown, employing the electricalbridge circuit principles described above in connection with Fig. 10.Referring to Fig. 12, the blockinglayer photocells 81 and 38 (Fig. 3)are indicated diagrammatically at 85 and 86 in two legs of an electricalbridge circuit. The photocells 85 and 86 are connected in series circuitrelation in one branch of the electrical bridge by a resistor 81, andthe other branch of the electrical bridge comprises a resistor 88. Theinput terminals of an amplifier, indicated diagrammatically at 89, areconnected between the electrical midpoints of the bridge circuit asindicated at 90 and 9|. Since the photocells 85 and 06 cannot ordinarilyas a practical matter be perfectly matched for internal resistance orcapacitance, an adjustable resistor 92 and a variable capacitor 93 areconnected in shunt circuit relationship with one of the photocells 85 or00.

The bridge circuit may be energized from a transformer, indicatedgenerally at 94, having its primary winding 95 connected forenergization from a 11S-volt alternating current source, and having asecondary winding 96 o! few turns. The secondary winding 96 is connectedacross the branches of the bridge circuit through an adjustable resistor91 whereby the voltage V impressed across the bridge may be regulated.The value of the voltage V is ordinarily adjusted to about 0.25 volt. o

The resistor 91 is a low resistant potentiometer rheostat having aresistance of the order of 1000 Vto 2000 ohms, the exact value not beingcritical.

The resistors 81 and 88 may be 100G-ohm potentiometer rheostats, and theresistor 92 may be a 10,000-ohm rheostat. rPhe exact values of theresistance 92 and the capacitance 93 depend upon the degree oi'difference in impedances of the photocells and 05. It will be understoodthat these details of the electrical bridge circuit are not critical andthat certain other arrangements may be used.

As stated, `the wave form pattern obtained with the electrical bridge iswell suited for direct control of /gas-fllled electric valves, of thetype commercially available as Thyratrons (General Electric) orGrid-glow tubes (Westinghouse). Accordingly, the output of the amplifier89 is capacitively coupled to the control grids of two such electricvalves |00 and |0I, as shown. The

raised to a certain maximum value (two or three to a value to which thebridge became sensitive. 'lo

grid bias is supplied and the firing points of the electric valves |00and |0| adjusted, by voltage from a battery indicated diagrammaticallyat |02 through suitable adjustable resistance, as

shown. Only low amplification is necessary, and

vthe amplifier 89 may be stage type.

The tube |00 is connected in series with one ofethe eld coils (notshown) of a reversible motor |03 while the other tube |i| is connectedin series with the other field coil of the reversible motor. The lamentsor heater elements of the electric valves |00 and |0| are connected forenergization to the secondary winding |06 of a of the conventionaltwotransformer |01. having its primary winding |08 connected across a11S-volt alternating current source. The -motor |03 may be of thedirectcurrent. fractional horse power type and has one terminal of one of itsfield coils connected to one side of the secondary Winding |04 of atransformer indicated generally at |05, while one terminal of the otherileld coil is connected to the opposite side of the secondary winding|04. Each of the other terminals of the field coils is connected throughone of the tubes or |0| and through the amature of the motor |03 to thecenter tap of the secondary winding |04.

It will be seen that the control grids of the electron tubes |00 and |0|are operated together A while the plates thereof are operated 180electrical degrees apart. As described in connectionwith Figs. and 11,small changes in light ilux to, or illumination'of. either of theblockinglayer photocells 85 and 86 will distort the residual-balancewave of the bridge circuit so as to produce either the wave form 83 or84 (Fig. ll), depending upon which of the photocells receives thegreater illumination. Accordingly, when the balance between theillumination of the photocells 85 and 86 is changed in one directionone' of the tubes |00 or |0| .will i'lre', while if the illumination isunbalanced in the opposite direction, the other tube will nre. Sinceeach of the field coils of the reversible motor |03 is connected withone of the electric valves |00 or |0|, the motor will run'in onedirection or the other depending upon which of the tubes is causedto re.Referring back to the operation of the concentration recorder a's showndiagrammatically in Figs. 3 and 4, it will be seen that the bridgecircuit will continue to be unbalanced and one of the tubes |00 or |0Iwill fire until the motor |03 has driven-the photometer member to apoint where the bridge circuit is again balanced. y

Employing the principles of my invention de,

scribed above, an ozone concentration recorder.

adapted to measure the concentration of ozone in air up to 1.0 per centwas constructed. The details of this recorder are described below inconnection with the remaining iigures of the drawings.

.Design of ozone recorder four compartments may be designated as "lightsource compartment ||5, "photocell compartment H6, recorder compartment|11, and

rectifier compartment" ||8. It will be noted l that the rectifiercompartment ||6 is that portionof the instrument which is under thepartition ||4 and back of the partition ||3. The recorder compartmentincludes that portion in front of the partition I3 and extends over therectifier compartment ||8. The side walls (not shown) of the instrumentmay be constructed with lightweight sheet steel, bent at the corners"Tand bolted to the steel frame. 'I'he back wall should be hinged so thatfree access to the recorder and rectifier compartments l1 and Ill may behad. The front wall should contain an l cording chart.

Referring to Figs. 13s, 14., and 16 of the drawings, it will be seenthat the vertical partition has a pair of beveled-edged upper and loweropenings or apertures and |2|, respectively. formed therein. A variableaperture, photometer member |22, constructed according to the theorydescribed above in connection with Figs. 5 and 6, is shiftably supportedon the right hand side (referring to Fig. 16) of the upper opening |24.The photometer member |22 is supported by a pair of upper and lowerflanged rollers |23, and an arm |24 fastened to the right hand endthereof which passes through a hole in the vertical Dartition ||2. Thearm |24 is supported and guided between a pair of guide rollers |25(Figs. 13b and 14h) mounted on the side of the partition A quartz-windowozone absorption cell |26 is mounted in front of the photometer member|22. The absorption cell |26 is supported by a pair of conduits |21which also serve to oon- `duct ozonated air to and from the absorptioncell |26 for circulation therethrough. A dummy, quartz-window absorptioncell |28 (Fig. 16) is supported in front of the lower window |2| by apair "of support members |29. Since the recorder is intended to measurethe-concentrations of ozone in air, it is not necessary to circulate airthrough the dummy absorption cell |20.

. and the two faces thereof ground so as to be parallel and as smooth aspossible. 'I'he reduced diameter nipples |34 entering the top of theframe f|33, may be soldered into the couplings |35 and the separator|33. No organic material should Abe present in the conduits l|2'| or theabsorption cell |26- since such material will cause a rapid absorptionof ozone andlead to an error in ready ing. 'I'he quartz-windows |36 maybe firmly held in place 'to the opposite sides of the separator |33 bypinch clamps (not shown), and the cell may be sealed by molten parafnpainted around' the edges thereof and allowed to solidify. The lowerabsorption cell |28 may be constructed in the same 'manner as theabsorption cell |26.

In order to adjust the width of the lower window |2| so as to adjust therecorder for zero readings, a shiftable shutter |40 is provided having asupporting arm |4| (Fig. 14s) extending therefrom. 'I'he arm |4| issupported between .a pair of capped pins |42 and the position of theshutter |40 may be adjusted by a screw |43 projecting into theinternally threaded end of the arm |4|. The head of the screw |43extends through the partition ||2 so that it may be reached with a screwdriver and the position of the shutter |40 adjusted by turning the same.A spring |44 compressed between the partition 2 and acollar |45 on theend of the arm |4| serves to securely bias andhold the shutter |40 inposition.

As show n in Figs. 13, l5 and 16, an M-shaped mercury vapor lamp may beused as a. source of 253.7 mmu radiation. The lamp |50 may be of thetype commercially available on the market as the WestinghouseSterilamp." 'I'he characteristics of the radiation of such a lamp havebeen described above in connection with the system of Fis. 2 of thedrawings. 'I'he lamp |55 is supported on a backing member v|5| by a pairof bands |52 (Fig. 15) which. are spring tensioned by a pair of tensionsprings |53 inserted between the baci; |l| and retaining washers |54, asshown in Fig. 13.. The backingmember |5| is supported from the bottomplate of the instrument box by a pair of uprights |55 fastened at theirlower ends in a pair of blocks |55. The front face of the backing plate|5|, which may be made of Bakelite, is smoked with magnesium oxide asindicated at |51 so as to substantially increase the amount of 253.7 mmuradiation radiated to the photometer part of the instrument.

An opaque metal screen |55 (Figs. 15 and 15) source excepting thosewhich proceed to the photometer assembly. The purpose of the screen |55is to prevent unnecessary deterioration of wire coverings and otherorganic material in the light source compartment ||5.

Referring again to Figs. 13., 14.. and 18 0f the drawings, it will beseen that the light oonversion and photocell system is mounted as a uniton the partition on the opposite side thereof from the photometerAmember |22 and the absorption cells-|25 and |25. The unit is enclosedin an outside band |55 fitting over four square posts I5! which projectfrom the partition The front of the Vbox is closed by a plate |52 havingopenings to accommodate a pair of upper and lower photocells |53 and|54, respectively. The photocells |53 and 54 are held in Place in theplate |62 by the flanges |55 and |55 thereof which engage the plate |52,and two sets of clips |61 (Fig. 13|). The photocelis |53 and |64 are ofthe blocking-layer, iron-selenium type. A pair of terminals |55 and |55project from the rear of each of the photocells |53 and |54,respectively.

Each of the photocells |53 and |54 is covered with a red filter |15. Theabsorption properties of the filters correspond to those of the redfilters I8 described in connection with Fig. 2 of the drawings. As'4stated, these filters may be of the type known as Wratten No. 25.

A pair of fluorescent glass plates 1| and |12 are mounted in front ofeach of the photocells |53 and |54, respectively, and supported in thethin metal frame |12.

cadmium borate phosphor and have the excitation characteristicsdescribedin connection with the iluorescent plate |1 of Fig. 2 of thedrawings. Horizontal separators |14 and 15 (Fig. 16) are provided in theunit to prevent fluorescent light from either of the fluorescent plates|1| and |12 from reaching the photocell |53 or |54 associated with theother fluorescent plate. The interior surfaces of the enclosing members|55 and |52 and the surfaces of the frame |13 and separators |14 and |15should be painted in flat black so as to minimize the e'ects of straylight.

From Fig. 16, it will be noted that the position of the mercury vaporlamp |55 -is so adjusted that the center thereof is at approximately thesame elevation as the center of the upper absorption cell and thephotometer, whereas the dummy absorption cell |25 is arranged at somedistance (approximately 2 inches) below the line of centers of the lampand the photometer. This arrangement is made'so as to obtain goodphotometric accuracy since the radiation should pass through theabsorption cell |25 and .photometer in as nearly parallel rays aspossible.

On the other hand, it is not a matter` of importance if the ra'diationfrom the lamp |55 strikes the lower fluorescent plate |12 at an obliqueangie after passing through the dummy absorption cell |25, and the lowerwindow |2|. 'Ihis is due to the fact that it is only necessary that thepercentage change in light ilux passing through the window |2| andincident upon the lower fluorescent plate |12 due to change in outputfrom the lamp |55, be the same as for the corresponding percentagechange in flux passing through the absorption cell |25, the photometer,incident upon the upper fluorescent plate There has been found to be anoptimum separation betweenV the upper fluorescent plate |1| .I and thephotometer member.|22. This optimum Ihe iluorescent plates |1| and |12are coated with a iilm or deposit ofy separation eliminates error andthe necessity for a perfect uniformity of the coating of cadmium boratephosphor on the plate. This will be understood on reference to Fig. 16where it will be seen that there is a wide divergence or spread of therays passing through any one of the three narrow elongated openings inthe photometer member |22. Thus, the. diagrammatic rays fan out inpassing through the photometer member |22, so as to cover approximatelyone-half of the area of the nuorescent screen or plate |1'|. This widedistribution and overlapping of the rays serves to reduce error due tovariation in thickness of the film of cadmium borate phosphor.

The reversible motor |55 for shifting the photometer member |22 inopposite directions, is shown in Figs. I3. and i4. mounted upon aplatform |5| supported by posts |52 extending upwardly from the bottomof the instrument box. The amature shaft |53 extends from the left handside of the motor |55 and carries a worm |54 which engages and drives aworm wheel |55. The worm wheel |55 is mounted on a horizontal shaft |55vsupported at one end in a bearing |51 and at the other end in a bearingin the vertical partition A pulley |55 is secured to the shaft |55adjacent the vertical partition over which a driving cord |5| runs. Oneend of the cord |5| is secured to the upper left hand corner of thephotometer member |22 as indicated at |52, while the other end of thecord is secured to the upper right hand corner of the photometer member|22 as indicated at |53. The driving cord |5| passes from |52 around thepulley |55 and thence around an idling pulley |54 carried on a pinprojecting from the upper end of an L-shaped lever |55. The pin on whichthe pulley |54l is supported passes through a window |55 cut through thevertical partition lil, and the lever |55 is pivotally supported to thepartition at |51. A spring |55 serves to bias the idling pulley |54toward the right and keep the cord |5| under a tension substantiallygreater than that required to move the photometer plate' |22 and therecording pen or stylus 255 (Fig. 13b) In this manner, friction isminimized and dimensional changes in the length of the' driving cord |5|are automatically compensated for.

Referring to Figs. 13b and 14s, for a description of the recordingapparatus, it will be seen that a synchronous motor 25| is mounted ontop of the rectifier compartment ||5. A small pinion gear 252 (Fig. 14s)is carried on the drive shaft of themotor 25| Qand meshes with a largegear wheel 253 mounted on one end of a shaft 254. The left hand end ofthe shaft 254 is Journaled in a bracket 255 which also supports thesynchronous motor 20|, and the right hand end is journaled in a block206 carried on a bracket 201. I'he shaft (204 also carries a worm 208which serves to drive a worm wheel 2| 0 keyed to the reduced end 2||(Fig. 13b). of a stubshaft 2|2. The 'stubshaft 2| 2 is journaled in abearing 2|3 carried on top of the bracket 201 as shown.

A collar member 2|4 having a large diameter' flange 2|5 is carried onthe iront end of the shaft 2|2. The flange 2|6 serves as aback supportto which a chart disc 216 is held. The I chart disc 2I6 fits over theend of the shaft 2 |2 -asodooi .i

, diagrammatically at 245, is connected across the and may be secured tothe ange,.2|5 by screws passing therethrough. Chart paper may be heldagainst the chart disc 2|6 by a large nut2l1. A

thin metal plate 220 having a large circular opening therein toaccommodate the chart .disc 2|6 is vertically supported around the disc2|6. Ears or tabs 22| are bent out from the thin sheet 220 at diierentpositions therearound and serve to retain a chart paper 222A in verticalposition on the disc 2 I6.

The synchronous motor drives the chart disc 2|6 at a constant speed andthe record of ozone concentration is made thereon by the stylus 223 ofthe recordingpen 200. The stylus v223 is carried in a receiver 224having a removable cap 225. A weak spring 226 within the receiver 224serves to press the stylus 223 gently against the chart paper on thechart disc 2|6. The re- -cording pen, 200 is adjustably mounted on theend' of the square arm 224 by a pin 221. The pin 221 isadjustable withina socket in the end of the arm 224 and may bel-,fastened in position bya-set screw 228. a

In order that the ozone concentration may be conveniently read at anytime, a scale 229 (Fig. 14h) is supported beneath the arm |24, and vapointer 40 is carried below the arm |24 so as to pass over the divisionsof the scale 229.

electrical mid-points of the bridgecircuit, as indicated.' .Theamplifier 245 may be energized from the 115-volt current source througha pair of conductors or leads 246 and 241. The output of the amplifier245 is capacitively coupled to the control elements or grids 248 and 249of a pair of gas-filled electric valves 250 and The n and are presentlyclassied as FG-98 (General scale 229 is calibrated in percentage ozoneconcentration from 0.0 tok 1.0 per cent, as shown.

, The electrical system iirst used in my'ozone4 concentration recorderis shown in Fig. 17 of the drawings, to which reference may be had for adescription thereof. The recording instrument may be energized from11S-volt alternating current source, as indicated. The synchronous moa16) are connected in one branch of an electrical bridge circuitcorresponding substantially to the electrical bridge described inconnection with Electric). The grid bias of the control elements 248 and249 may be adjusted from a B bat.

tery indicated diagrammatically at 252, Vthrough suitable adjustableresistance. The heated iliaments'256 and 261 of the electric valves areconnected for energization with the secondary 258 of a transformerindicated generally atf269.

One of the eld coils 260 of the reversible motor is connected in seriescircuit relation with the electric valve 260, while the other iieldvcoil26| is connected in series circuit relation with the other electricvalve 25|. As will be seen, the circuit for the eld coil 260 iscompleted through one-half of the secondary winding 262 of thetransformer 259, the armature of the motor |80,Ik and the tube 260, thearmature being connected to the center-tap of the secondarywinding 262.

The circuit for the field coil 26| is completed through the other halfof the secondary 262, the

larmature of the motor |80, and the other tube 26|. The primary winding263 of the transformer 269 is connected for energization across the11'5- volt source, as shown. A toggle switch 264 is prof cept for theSterilamp lamp |60 and the synchronous recorder motor 20 The fourthelements 266 and 266 of the tubes 260 and 26| serve to give steadier`operation of the electrical system. It will be seen that the groundoutput terminal of the ampliner 245, the fourth tube elements 266 and266, and the center taps of the secondary transformer windings 2 68 and-262.

are interconnected or grounded.

In operation of my ozone concentration recorder, the electrical circuitcontrol system is first adjusted, conveniently with the help of an oscilFig. l12. One branch of the bridge comprises the two photocells |63 and|64 interconnected in series circuit relation by a resistor 236, and thel'other branch comprises a potentiometer type rheostat 236. The twobranches of the bridge are connected in parallel across the terminals ofthe secondary winding 231 of a step-down transformer 238. The primarywinding 239 is connected for energization across the 115-volt line, asshown. The transformer 238 is approximately the size and capacity of asmall door-bell ringing transformer, andra secondary 231 consists ofonly a few turns so that the voltage V impressed across the triagecircuit win be inthe order of 0.25 76 loscope.

With the mercury vapor lamp |60 turned oil, the electrical bridgecircuit including the photocells 68 and |64 is balanced so as to obtaina balanced residual wave fopn as describedl in connection with Figs. 10and 11 of the drawings. Then, the mercury vapor lamp |60 is `turned onand with the absorption cell |26 entirely free of ozone, the width ofthe lower window |2| is adjusted with the shutter |40 so that theVsystemdncluding the reversible motor |80 through the absorption cell|28 by suitable pumping apparatus. 2517 mmu radiation is ab` sorbed bythe ozone on passing through the cell |20, the system becomes unbalancedand one -of the tubes 25| or 25| will fire over a longer portion of thecycle than the other. Accordingly,

the reversible motor i will rotate in such a direction as to shift thephotometer member |22 to a position permitting more 253.7 mmu radiationto pass and thereby bring the system back into balance. Conversely, inthe event that the concentration of ozone in the absorption cell |25decreases, thereby permitting an increased amount of 253.7 mmu radiationto pass, the ilrinsl period of thetubes 2l. and 25| will again becomeunbalanced, but in an opposite direction, so as to rotate the motor |50in an opposite direction. In turn, the photometer member |22 will beshifted so as to decrease the amount of radiation passing darkness, theresidual wave When vthe phototubes 215 and 21| are in total may be madezero; then.' when the light level on the tubes is brought up andbalanced, the residual wave comes into existence, and increases withincreasing light evel.

The use of phototubes of the photo-emissive type as described inconnection with Fig. 18, has

.led to much more satisfactory operation of the recorder, sincesensitivity is appreciably greater and the stability or freedom fromdrift has been greatly increased. Furthermore, it is easier to obtainphototubes of the photo-emissive type which are matched because it isonly necessary that these tubes be similar inthe current-voltagecharacteristic, whereas photocells of the blocking-layer type must matchin characteristic, internal resistance and capaci- I tance.

through the photometer and bring the system back in balance.

As described, the motion of the variable aperture, photometer member |22in following the concentration of ozone in theabsorption cell I2! istranslated to the recording pen 208. In this manner, the pen makes arecord of the ozone concentration on the chart paper 222 (Fig. 14s).

Althoughmy ozone concentration recorder, as

described abve, operated satisfactorily in a prac- Y tical manner, itwas found that even much more satisfactory operation of the recordercould be obtained by modifying the electrical bridge of the electricalcontrol system of Fig. 17. This modification consisted in substitutinglight-sensitive tubes or photocells of the photo-emissive type (vacuumor gas-filled phototubes) for the photocells Il! and l (Fig, 17) which,as stated, were of the blocking-layer type.

The modified bridge circuit is 'shown diagrammatically in Fig. 18 of thedrawings as electrically interconnected between a transformer 210 andthe amplifier 24| (Pig. 17). The transformer 21| corresponds infunctionto the transformer 22| (Fig. 17), but the transformer ratio thereof isgreater than that of the transformer 232 so as to impress a greatervoltage across the bridge circuit. Whereas it was found that the bridgecircuit of Fig. 17 had its greater sensitivity when a voltage in theorder of 0.250 was impressed across it, it has been found that thebridge circuit of Fig. 18 should have an impressed voltage in the orderof 3 to 5 volts.

One branch of the modified bridge circuit of Fig. 18 is provided with apotentiometer type rheostat 212 while the other branch of the bridge iscomprised of two phototubes 214 and 215 of the photo-emisslve typeinterconnected in series circuit relation. A small variable capacitor211 is connected in shunt relationship with each of the phototubes 214and 215, as shown, so that any slight difference in capacitance of thetwo tubes maybe corrected. The'lnput of the'clectronic amplifier 245 isconnected across the electrical mid-point of the bridge circuit. as indicated. Except for the modification of the electrical bridge asdescribed, the electrical control system by which the Thyratrons orgas-illled valves 25| and 25| are controlled is the same as the controlsystem shown in Fig. 17.

The residual wave obtained with the bridge of Fig. 18 is very similar tothat observedwhen blocking-layer photocells are employed (Fig. l1),

although the dependence of the amplitude of the residual wave on lightlevel is much stronger.

As another modification of the electrical bridge of the invention, thephotocells employed therein may be a pair of the early type seleniumcells. This type of cell is of the photo-conductive typo and involvesordinary conduction through a thin fllmof selenium, which exhibits achange in resistance when illuminated.

Although I have described an embodiment of my invention specificallyadapted to measure and record concentrations of ozone in air, it will beunderstood that modifications and adjustments may be made so that byapplying the same principles of invention, instruments may be made formeasuring and recording concentrations of other gases and vapors, aswell ascertain solutes in solvents.

It will be understood that in apparatus of this nature, involving as itdoes a relatively large Anumber of parts and elements organized intodifferent systems, vcertain changes, modifications and otherarrangements may be made without departing from the-principles and scopeof the line of the mercury spectrum; a cadmium borate phosphorfluorescent plate disposed to receive radiation from said mercury-vaporlamp, the excitation characteristics of said fluorescent plate beingsuch that it is very strongly excited by ultraviolet radiation of the253.7 mmu line of the mercury spectrum according to the intensitythereof. receiving substantially no excitation at all from visiblelight, and only weakly excited by near-ultraviolet light, the strongexcitation of said fluorescent plate by said concentrated ultravioletradiation of the 253.7 mmu line of the mercury spectrum causing it tofluoresce in the orange and red region of the spectrum; and a -filterdisposed to receive and transmit fluorescent radiation from saidfluorescent plate, the absorption'characteristics of said filter beingsuch that visible light in the orange and red region of the spectrum dueto said strong excitationy of said fluorescent lplate is transmittedcurrent-voltage.

